1. Field of the Invention
The present invention relates to an anode including an anode active material layer on an anode current collector, and a battery including the anode.
2. Description of the Related Art
In recent years, portable electronic devices such as camera-integrated VTRs (videotape recorders), cellular phones, or laptop computers are widely used, and size and weight reduction in the portable electronic devices and an increase in longevity of the portable electronic devices have been strongly demanded. Accordingly, as power sources for the portable electronic devices, the development of batteries, specifically lightweight secondary batteries capable of obtaining a high energy density have been promoted. Among them, a secondary battery (a so-called lithium-ion secondary battery) using insertion and extraction of lithium for charge-discharge reaction holds great promise, because the secondary battery is capable of obtaining a larger energy density, compared to a lead-acid battery or a nickel-cadmium battery.
The lithium-ion secondary battery includes a cathode, an anode and an electrolytic solution, and the anode includes an anode active material layer on an anode current collector. As an active material (anode active material) of the anode, a carbon material is widely used; however, recently with enhancement of performance and expansion of functions in portable electronic devices, a further improvement in battery capacity is desired, so it is considered to use silicon instead of a carbon material. It is because the theoretical capacity of silicon (4199 mAh/g) is much larger than the theoretical capacity of graphite (372 mAh/g), so an increase in battery capacity is expected.
In the case where silicon with high theoretical capacity is used as an anode active material, some attempts have been made. More specifically, a technique in which a conductive metal is reductively deposited on silicon particles (for example, refer to Japanese Unexamined Patent Application Publication No. H11-297311), a technique in which a silicon compound is coated with a metal (for example, refer to Japanese Unexamined Patent Application Publication No. 2000-036323), a technique in which a metal element not alloyed with lithium is dispersed in silicon particles (for example, refer to Japanese Unexamined Patent Application Publication No. 2001-273892), a technique in which copper is dissolved into a silicon thin film (for example, refer to Japanese Unexamined Patent Application Publication No. 2002-289177) or the like has been proposed.
As a method of forming the anode active material, a vapor-phase method such as a sputtering method is used (for example, refer to Japanese Unexamined Patent Application Publication Nos. 2005-285651, 2006-278104 and 2006-086058). More specifically, silicon is deposited on an anode current collector having a rough surface configuration, thereby an anode active material is formed so that the anode active material includes a plurality of columnar sections separated by gaps in a thickness direction, and each columnar section is in close contact with an anode current collector.
However, when silicon as an anode active material is deposited by a vapor-phase method, a plurality of pores (holes) are formed in the anode active material to increase the surface area of the anode active material, and the anode active material into which lithium is inserted during charge has high activity, so an electrolytic solution is easily decomposed, and lithium is easily inactivated. Thereby, while the secondary battery obtains a higher capacity, cycle characteristics which are important characteristics of the secondary battery easily decline. In particular, the presence of pores with a small diameter has a large influence on the surface area of the anode active material, so the presence of the pores is a major cause of a decline in cycle characteristics.
Some cases about the formation of a plurality of pores in the anode active material have been reported (for example, refer to Japanese Unexamined Patent Application Publication Nos. 2005-293899 and 2004-071305). In this case, the case where during initial charge and discharge, a plurality of primary particles are aggregated to form a plurality of secondary particles, and the secondary particles are separated by grooves in a thickness direction, and some of the primary particles are split by the grooves to form split particles has been reported (for example, refer to Japanese Unexamined Patent Application Publication No. 2006-155957).
Like the above-described pores with a small diameter, the widths of grooves separating secondary particles have a large influence on cycle characteristics. More specifically, when a large number of grooves with a narrow width which separate secondary particles per unit area are formed, the size and the surface area of one secondary particle are reduced; however, the surface area of the whole anode active material made of secondary particles is increased, so an electrolytic solution is easily decomposed. On the other hand, when grooves with a wide width which separate secondary particles are formed, secondary particles are less likely to be split into small pieces, so the surface area of the anode active material per unit area is reduced, so the electrolytic solution is less likely to be decomposed, but the size of one secondary particle is increased, so a stress according to swelling and shrinkage of the anode active material during charge and discharge is increased. Thereby, it is difficult to release a stress in the anode active material layer, so an anode active material layer is easily peeled from an anode current collector.